The invention relates to a fuel supply pump of the type generally disclosed in our earlier U.S. Pat. No. 4,205,947 and assigned to the assignee of this application. Fuel supply pumps have already been proposed which have a pumping system comprising a first supply pump and a main supply pump connected in seriatim which is driven as a whole by a common electromotor. With these known fuel supply pumps, there are difficulties associated with trying to achieve a trouble-free coupling between the two pumping stages, without the moving parts of both pumping stages, through their separate motion, influencing each other too strongly. In this respect it is of particular importance that a trouble-free sealing off of the pumping stages from each other be accomplished in the area of the bearing of the rotating parts, that is, a sealing off of the high pressure space from the medium pressure space.